Process for dehydrogenation



United States Patent 3,290,406 PRGCESS FOR DEHYDROGENATION William C. Pfelferle, Middletown, N.J., assignor to Engelhard Industries, Inc, Newark, N.J., a corporation of Delaware N0 Drawing. Filed June 2, 1965, Ser. No. 460,835

4 Claims. (Cl. 260683.3)

This invention relates to a process for the dehydrogenation of hydrocarbons, and is a continuation-in-part of copending application, Serial No. 196,552, filed May 15, 1962, now abandoned.

In accordance with the aforesaid copending application, gas phase reactions wherein hydrogen is a product of'the reaction are effected in the presence of a membrane which is selectively permeable to hydrogen and the hydrogen produced during the reaction is continuously removed from the reaction zone by permeation through such selectively permeable membrane. As a result of the continuous separation of hydrogen from the reaction zone, it becomes possible to utilize more economical reaction conditions, including by way of illustration, adjustment of. reactant concentrations, to obtain improved product yields and conversions as compared to conventional' operations under comparable conditions in the absence of hydrogen separation.

While the process described and claimed in the aforesaid copending application is applicable, in general, to a wide variety of gas phase reactions wherein hydrogen is produced as a reaction product, the present invention is particularly concerned with, and directed toward, an improvement in the process for dehydrogenating hydrocarbons.

The dehydrogenation of hydrocarbons is an important reaction in commercial chemical and petrochemical operations. For example, normal butane is dehydrogenated to butenes, normal butane and butenes are dehydrogenated to form butadiene, isoamylenes are dehydrogenated to form isoprene and ethylbenzene is dehydrogenated to form styrene. Such dehydrogenation can be effected in the presence or absence of a catalyst, although catalytic dehydrogenation is generally employed in commercial processes.

Such dehydrogenation reactions are endothermic and thus have more favorable equilibrium constants at higher temperatures. For example, for the dehydrogenation of propane:

1400 F., K =5.2 (approx) l200 F., K =0.85 (approx) 1000 F., K =0.08 (approx.)

It can be seen that 1400* F. is a far more favorable operating temperature than 1000 F. to achieve maximum conversion of propane. However, higher temperatures also favor thermal cracking reactions and coking, while lower temperatures favor the ratio of dehydrogenation over cracking, reduce the coke formation, and permit the use of more active and more selective catalysts. For example, in a typical conventional method for producing propylene by dehydrogenation of propane, a temperature of 1490 F. and a pressure of 9 p.s.i.g. are employed. No catalyst is used because of the high temperature.- Although about 60 to 75 percent of the propane is converted in this thermal dehydrogenation process, it can be estimated that the yields are poor because of the competing thermal cracking reactions which high temperatures favor.

In order to overcome the disadvantages of higher temperatures and achieve greater selectivity in the reactions,

conventional methods must operate at very low pressures. For example, in a typical method described for the catalytic dehydrogenation of butane (for which reaction equilibrium constants indicate that somewhat lower temperatures can be employed than in the dehydrogenation of propane), the operating temperature of 1150 P. required that a pressure of 3 p.s.i.a. be used. This means that in order to operate at lower temperatures, either a diluent or vacuum must be employed, which adds to the cost of the process. In addition, only a 9-minute cycle life was reported as commercially attained. Under these conditions, a waste of 7 percent results for every 10 percent of butadiene product. This waste can be attributed in part to the lack of selectivity of the catalyst used under these conditions.

Using the K for propane to propylene as given above, at 1400 F. and 3 atmospheres, less than about 79 percent oonversion of the propane is obtained using conventional techniques. However, if the partial pressure of the hydrogen in the reaction zone is reduced to 0.3 p.s.i., the same percent conversion of propane can be obtained at 1000 F.

In accordance with the present invention, desirable low partial pressures of hydrogen are achieved in the dehydrogenation of hydrocarbon feeds by effecting the dehydrogenation while continuously separating hydrogen.

from the reaction zone by diffusion through a non-porous membrane selectively permeable to hydrogen, and reacting the diltused hydrogen withoxygen on the downstream side of the membrane in order to substantially instantaneously reduce the partial pressure of hydrogen on the downstream side of the membrane to zero. Since the rate of hydrogen diffusion through the membrane is a The process of the present invention ofl ers numerous advantages in the dehydrogenation of hydrocarbons. For example, such dehydrogenations are ordinarily eflfected in the presence of substantial quantities of inert gases,

such as carbon dioxide or nitrogen in order to reduce the hydrogen partial pressure. When steam is employed as a gaseous diluent, it serves the dual purpose of reducing the hydrogen partial pressure and also facilitates the removal of carbon from the catalyst. When the dehydrogenation is effected in accordance with the present invention, the amount of such diluent gases can be substantially reduced, or even completely dispensed with, thus increasing the capacity of the dehydrogenation reactor as well as providing economic advantages in reducing the heat requirements of the system. Further, hydrocarbon dehydrogenations are normally effected at atmospheric pressure, or slightly above, in order to maintain the lowest possible hydrogen partial pressure. The present invention permits such dehydrogenation reactions to be effected at more elevated pressure, for example up to p.s.i.g. or even higher, because of the substantial reduction of hydrogen content in the reaction zone due to permeation of the hydrogen through the hydrogenpermeable membrane. Another advantage of the present process is to supply the heat to the dehydrogenation zone by the exothermic reaction of hydrogen and oxygen at the surface of the hydrogen-permeable membrane. Since dehydrogenation reactions are highly endothermic, the heat of the hydrogen combustion substantially reduces the input heat requirements of the system.

The process of the present invention can be employed for the dehydrogenation of various types of dehydrogenateable aliphatic, including cycloaliphatic, hydrocarbons, containing from 2 to 20 or more carbon atoms which may Patented Dec. 6, 1966 3 4 be cracked during the process. Suitable parafiins include process of the present invention since hydrogen diffuses for example branched or straight chain paraffins of about through these materials at realistic commercial rates at 4 to 8 carbon atoms, cyclic paraflins such as cyclopentane temperatures ranging from about 200 C. to 1100" C., or cyclohexane and cycloparafiins having an aliphatic side which temperature range broadly encompasses the effecchain that can undergo dehydrogenation as for example tive temperature range for the dehydrogenation reaction. ethylcy-clohexane to styrene. Aromatics containing ali- In effecting the dehydrogenation, the hydrogen formed phatic substituents such as ethyl, n-propyl, isobutyl, in the reaction zone and which passes through the n-butyl groups can be employed in the process of this hydrogen-permeable membrane is reacted with oxygen invention. The method of the present invention is of by contacting with a gaseous mixture containing oxygen, particular value in the dehydrogenation of branched, 10 e.g., air, mixtures of air or oxygen with an inert diluent straight chain or cyclic olefins containing about 4 to 8 such as nitrogen and the like. The particular concentracarbon atoms and which undergo dehydrogenation to tion of oxygen is not critical, but will be adjusted in diolefins. The method can be employed for the producorder to effect the combustion of hydrogen at a rate tion of butadiene and/ or isoprene from the dehydrogenawhich will provide desirable temperatures and avoid tion of normal butylenes and isoamylenes. The dehydroover-heating of the reactor. For example, pure 0 or genation of these and other dehydrogenateable hydroair may be used. Preferably 'theoxygen concentration carbons is well known in the art. will range from 0.1 to about 50% by volume. The The reaction of the present invention can be effected oxygen-hydrogen reaction can be effected at atmospheric under conventional dehydrogenation conditions, the exact or superatmospheric pressure, for example up to about set of conditions being dependent on the feedstock and 200 p.s.i.g. It will be appreciated that the palladium or product desired. Generally, temperatures in the range palladium alloy membrane serves in this regard as a cataof about 800 to 1300 F. and pressure from about 0.05 lyst to promote the combustion of oxygen (palladium is to 200 p.s.i.g., preferably from atmospheric to 100 p.s.i.g., a well known catalyst for this reaction), and the oxidaand weight hourly space velocity (WHSV) about 0.01 tion is further enhanced by the fact that the diffused to 100, preferably 0.1 to 10. The reaction may be ofhydrogen is probably in the atomic form as it emerges fected in the presence of any inert diluent gas such as from the palladium surface. Thus the combustion reacnitrogen, methane, etc., to further reduce the partial prestion is extremely fast, and hydrogen is substantially insure of the hydrogen in the dehydrogenation reactor zone. stantaneously consumed on the downstream side of The dehydrogenation can be eflfected thermally, or in the membrane.

the presence of conventional dehydrogenation catalysts The following example is included to further illustrate well known to the art. Suitable catalysts used in the art the present invention.

of dehydrogenation of hydrocarbons include Standard Oil Development Company 1707 which is a combination of magnesium oxide, ferric oxide, chromium oxide and In thls example a commerclal hydrogen dlfliuslon potassium oxide, Dow Type B which is a combination of (Engelhard Dlfillser q conslstmg of a lackfited nickel calcium phosphate and chromium oxide, and the assembly of f PanadmIn alloy tubes (Palladmm' like. Supported platinum group metal catalysts, e.g., Percent saver) was employed as i dehydrogenaplatinum on a solid, non-acidic refractory metal oxide, reactor Ethane Was to the dlffuslon tubes at have been proposed, and may be employed, for dehydro about 125 p.s.1.g., the palladium alloy tube servlng as a genation in accordance with the present invention. dshydwgenafion catalyst The dlficuslon tubes f Where the dehydrogenation is effected in a tubular reacmamtamed at a temljeramre' of about 700 to 9 tor consisting of palladium or palladium alloy, the reacon h downstream slde of f a gaseous Imxture tor will itself serve to a certain extent as a dehydrogenaof nltfogen and OXYgeH contalnlng l 2 i catalyst, was fed at a pressure of about 100 p.s.1.g. A sample of The dehydrogenation of dehydrogenateable hydmthe reaction effluent was collected during the 15 to 18th carbons is effected in accordance with the present invenhours of Operation, analyzed by IR and found to contain tion by conducting the dehydrogenation in a reaction 0.7% ethylene. The following table shows details in zone from which hydrogen is continuously removed by this example.

Example TABLE Hours on Stream -l 0/2 0/3 6/9 9/12 12/15 15/18 18/21 21/24 24/25 Temp., F 703 851 850 849 849 848 845 849 851 Reactor Pressure, p.s.i.g 123 124 124 124 124 124 124 124 124 Oxidation Zone Pressure, p.s.i.g 93 100 100 102 101 100 100 103 103 M01. Ratio Oz/CzH 1. 3 98 86 86 1. 03 1. 03 90 87 90 Ethylene, percent in effluent 0. 5 0. 58 0. 71 0. 6 0. 66 0. 70

permeation through a non-porous membrane selectively 60 It will be appreciated that the dehydrogenation of permeable to hydrogen. For example, the reaction can be effected within a tubular reactor constructed of a hydrogen permeable material such as palladium or palladium-containing alloys. Where a catalyst is emethane was effected at a relatively low temperature, in this example, but nevertheless substantial dehydrogenation was obtained even at these unfavorable conditions. With suitable adjustments in the reaction conditions, the

ployed to promote the dehydrogenation reaction, the yield of ethylene can be substantially improved.

tubular reactor can be packed according to conventional techniques with such catalyst in pellet, pilled or other suitable form.

Suitable materials for the removal of hydrogen formed What is claimed is:

1. In a process for the dehydrogenation of dehydrogenateable hydrocarbons is a reactor maintained under dehydrogenation conditions which preclude any substanin the reaction zone are palladium and palladium alloys, tial thermal cracking of the hydrocarbons including a e.g., palladium-silver alloys of the type disclosed in US. Patent No. 2,773,561. Such materials provide rapid diffusion of essentially completely pure hydrogen, the diffused gas containing less than 1 p.p.b. impurities.

temperature of about 800 to 1300 F. and a pressure in the range of 0.05 to 200 p.s.i.g., the improvement which comprises continuously removing hydrogen formed during the dehydrogenation from the reaction zone by diffu- Palladium and its alloys are particularly useful in the sion through a non-porous membrane selectively permeable to hydrogen, and contacting the diffused hydrogen with oxygen-containing gas to effect substantially complete combustion thereof at the surface of the membrane.

2. The improved process of claim 1 wherein the feed is a dehydrogenateable aliphatic hydrocarbon of 2 to 20 carbon atoms.

3. The improved process of claim 1 wherein the nonporous membrane is composed of a material selected from the group consisting of palladium and palladiumcontaining alloys.

4. The improved process of claim 1 wherein the oxygen-containing gas contains from about 0.1 to 50 volume percent oxygen.

References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS 7/1959 Canada.

DELBERT -E. GANTZ, Primary Examiner.

S. P. JONES, Assistant Examiner. 

1. IN A PROCESS FOR THE DEHYDROGENATION OF DEHYDROGENATABLE HYDROCARBONS IS A REACTOR MAINTAINED UNDER DEHYDROGENATION CONDITIONS WHICH PRECLUDE ANY SUBSTANTIAL THERMAL CRACKING OF THE HYDROCARBONS INCLUDING A TEMPERATURE OF ABOUT 800 TO 1300*F. AND A PRESSURE IN THE RANGE OF 0.05 TO 200 P.S.I.G., THE IMPROVEMENT WHICH COMPRISES CONTINUOUSLY REMOVING HYDROGEN FORMED DURING THE DEHYDROGENATION FROM THE REACTION ZONE BY DIFFUSION THROUGH A NON-POROUS MEMBRANCE SELECTIVELY PERMEABLE TO HYDROGEN, AND CONTACTING THE DIFFUSED HYDROGEN WITH OZYGEN-CONTAINING GAS TO EFFECT SUBSTANTIALY COMPLETE COMBUSTION THEREOF AT THE SURFACE OF THE MEMBRANE. 